The area of thin film batteries, particularly those employing lithium, has seen great advances over the past several decades. In addition, the recent increases in the price of conventional energy production have focused attention on alternative energy sources. In particular, the photovoltaic solar cell has been improved significantly and its cost has been reduced. At the same time, prices for fossil fuels have risen to the point where it is projected that solar power will be comparable in cost to grid power by 2010. Today's second generation solar cells based on thin films of amorphous and microcrystalline silicon deposited on large glass substrates offer the most promise to achieve cost parity to grid power within a few years.
A limitation of photovoltaic devices is that they are only able to provide power when illuminated. This requires devices that must operate at other times to have an alternative source of power. In the case of mobile electronics, automobiles, etc. that cannot conveniently be plugged into mains outlets, the simplest form of backup power is an electrochemical cell or battery. Electrochemical cells provide an excellent combination of energy capacity, power density and economy. Some cell designs can be recharged many times—so-called secondary batteries. Currently, thin-film lithium batteries offer the best performance in terms of energy density, power density and cycle life. Like the second generation solar cells, they are also fabricated using thin-film deposition techniques.
Many individuals have made prior attempts to provide solutions and improvements to the issues surrounding the design and production of photovoltaic cell/thin film battery combinations. For example, in 1984, U.S. Pat. No. 4,481,265 was issued to Ezawa et al. describing a “Photovoltaic-Storage Battery Device.” This patent disclosed an insulating substrate with photovoltaic cells on one side and a battery on the opposite side, though the inventors also described how both devices could be incorporated onto the same surface. The description of the device described a battery that used a liquid electrolyte but their claims include a battery that includes an inorganic film electrolyte. In 1984, the then state-of-the-art solid-state batteries provided far inferior performance to batteries with liquid electrolytes. It was left to the reader to determine how to fabricate the all solid-state device and what materials to use. The overall manufacturing method described was complicated and inelegant.
In 1988, U.S. Pat. No. 4,740,431 granted to Little described a far more elegant and practical method for fabricating an integrated solar cell and battery. This inventor disclosed a process whereby a solar cell and a battery were deposited solely by depositing and patterning thin films onto a substrate using techniques pioneered in the semiconductor industry. At that time, glasses based on lithium sulfide appeared to be the most promising solid-state electrolytes for all solid-state batteries.
However, in 1994, Bates et al. described a superior solid state electrolyte based on lithium phosphorus oxynitride (LiPON) and were granted U.S. Pat. No. 5,338,625. In addition to disclosing the new thin-film solid state electrolyte material, the authors claimed a thin film battery that they described as a thin-film microbattery and that was deposited using thin-film deposition methods. Following this pivotal discovery, many inventors were granted patents that sought to improve various aspects of the thin-film microbattery.
There were at least two major drawbacks to the thin-film microbattery disclosed by Bates (and also common to the battery described by Little) which stemmed from the fact that the battery was fabricated entirely from thin-films deposited onto a substrate that was not electrochemically active. First, the capacity per unit area of the disclosed thin-film batteries is very low because the electrodes are very thin. Second, the fabrication cost is very high compared to batteries fabricated from bulk materials due to the need for expensive vacuum deposition equipment.
An innovative way to solve the problem of poor capacity was proposed by Hobson and Snyder in U.S. Pat. No. 5,445,906 (issued 1995) whereby the films were deposited onto a large area flexible substrate in a web coater. The substrate was then wound into a tight spiral, thereby achieving large capacity per unit volume. However, use of a web coater further increases the manufacturing costs and the use of a tightly wound spiral battery is not easily integrated with a large planar thin-film solar cell.
More recently Jenson, in U.S. Patent Application No. US 2004/0185310, has disclosed a “Method and Apparatus for Integrated Battery-Capacitor Devices” which attempts to provide an integrated battery-device solution. The Jenson reference teaches depositing the battery onto a conductive substrate layer which is a separate structural component from the battery. Jenson also discloses integrating the proposed battery with a photovoltaic cell, although all permutations taught include the use of a substrate which is a separate and distinct structural component from the battery.
Thus, although the Jenson reference touches on both thin film batteries and photovoltaic cells, it fails to embrace the invention contained herein: a thin film battery with a self supporting cathode or anode layer which can be more easily and economically manufactured than the batteries present in the prior art. In one embodiment of the present invention, the self supporting layer is significantly thicker (on the order of 50 times) than previously taught resulting in dramatic increases in the capacity per unit area of the cell as compared to traditional thin film cells. Additionally, the addition of an integrated solar cell in the present invention provides an elegant solution to design issues present in thin film battery evolution while saving space, weight and increasing speed and ease of manufacture.